The invention relates to a computed tomography method which includes the steps of: generating, using a radiation source, a radiation beam which traverses an examination zone or an object present therein, generating a relative motion, including a rotation about an axis of rotation, between the radiation source on the one side and the examination zone or the object on the other side, acquiring measured values for a plurality of radiation source positions by means of a detector unit which is coupled to the radiation source and includes at least one row of detector elements.
The invention also relates to a computed tomography apparatus for carrying out the method.
The number of read-out channels in a computed tomography apparatus is limited because of the costs of the data acquisition, the transmission bandwidth of the data slip ring and/or the data throughput that can be handled by a reconstruction unit. For example, in the case of a computed tomography apparatus which rotates around an examination zone three times per second with a detector unit which acquires data from 1400 different angular positions during each rotation and comprises 16 rows of 1000 detector elements each, the measured values must be transferred and evaluated with a transmission bandwidth of approximately 200 Mbyte/second (assuming that a measured value of 3 bytes is transferred). This would only be possible, if possible at all, at high costs.
GB 2,005,955 already discloses a computed tomography apparatus of the kind set forth in which the signals from two or more detector elements at the edge of the detector unit are combined. Thus, fewer measured values need be transferred to the reconstruction unit reconstructing computed tomograms from the measured values; however, this reduction is achieved at the expense of the spatial resolution at the edges of the examination zone at least.
Such a combination of signals would lead to a further reduction of the resolution in computed tomography apparatus including the meanwhile customary so-called xe2x80x9cquarter detector shiftxe2x80x9d (where the detector unit is arranged relative to the radiation source and the axis of rotation in such a manner that the projections of the axis of rotation on the detector unit are shifted one quarter of the width of a detector element relative to the center of the detector unit), because the advantages associated with the quarter detector shift can no longer be utilized.
It is an object of the present invention to propose a computed tomography method of the kind set forth such that a more attractive compromise is reached between the number of measured values to be measured/transmitted and/or processed on the one hand and the image quality on the other hand.
On the basis of a computed tomography method of the kind set forth this object is achieved according to the invention in that the method also includes the following steps:
combining the signals of each time at least two neighboring detector elements so as to form one measured value, and
cyclically varying the combinations from one radiation source position to another.
The invention is based on the following considerations.
Each detector element in a detector unit has several neighboring detector elements. Thus, the output signals of the detector elements can be combined in various manners. As has already been explained, each combination per se would lead to a reduction of the resolution. However, if dynamic switching over takes place from one of said combinations to another upon changing over from one radiation source position to the next position in such a manner that the combinations are cyclically run through, only a small loss of spatial resolution will occur. This is because the combinations supplement one another in such a manner that the measured values are distributed in the Radon space in substantially the same way as in the absence of these signal combinations.
A computed tomography apparatus for carrying out the method according to the invention includes a radiation source for generating a radiation beam which traverses an examination zone or an object present therein, a drive unit for realizing a relative motion, including a rotation about an axis of rotation, between the radiation source on the one side and the examination zone or the object on the other side, a detector unit which is coupled to the radiation source and includes at least one row of detector elements for the acquisition of measured values for a plurality of radiation source positions, wherein the apparatus includes a combination device for combining the signals of each time at least two neighboring detector elements so as to form one measured value, and means for cyclically varying the combinations from one radiation source position to another. An alternative embodiment also concerns a CT apparatus with a so-called cone beam. Even though the advantages of the invention are also achieved when the detector unit has one row only (fan beam CT), the invention has more effect for cone beam CT than for fan beam CT.
Yet another embodiment describes a first possibility for the combination of detector elements. This combination of the signals from neighboring columns (but the same row) is to be used preferably in conjunction with the so-called quarter detector shift. In a separate embodiment, however, the signals from detector elements of neighboring rows can also be combined. The embodiment may also be used in combined form.